The integral double-layer capacitance Cdl,i can be calculated by using the GouyChapman-Stern model. In this model, the double-layer capacitance consists of a series network of a Helmholtz-layer capacitance (the Stern capacitance) and a diffuse-layer capacitance. The Helmholtz layer models the effect that the ions in the solution have a finite size and the centres of the ions cannot approach the surface any closer than the ionic radius including a possible water layer which means that there exists a plane of closest approach for the centres of the ions at some distance, xH. The diffuse layer, starting from xH, contains the same amount of charge (of opposite sign) as the oxide surface charge, because the Helmholtz layer is by definition not containing any charge. The charge in the diffuse layer σdl is given by